With the development of superconductor technology, it becomes feasible to install superconductive fault current limiters in a power grid used to limit the short-circuit fault current. At present, among all kinds of superconductive fault current limiters developed all over the world, the core-saturated superconductive fault current limiter is most desirable due to the following properties: incorporating detection, initiation of limiting action and current limitation together, and no quench of the superconductivity in the process of current limiting.
The traditional core-saturated superconductive fault current limiter is composed of four parts: an iron core group, an AC winding, a superconductive magnet (namely a superconductive winding), and a direct current (DC) power supply. The superconductive magnet provides excitation for two parallel iron cores. Two AC coils connected in series are wound on two iron cores respectively to cancel the magnetic fields generated in the center column so as to minimize the effect of the AC inductance voltage on the DC superconductive windings. When the current limiter operates normally, the DC excitation causes the iron cores to be in the deep saturation. Iron cores produce small inductive impedance in the AC winding so that there is no effect on the power grid. In the state of the fault current limiting, the super high short-circuit current drives one iron core out of saturation in a half-wave and the magnetic field in the other iron core increases to realize the current limiting by a single iron core (the reactor in the enhanced magnetism state is not engaged in the current limiting). This is so-called passive fault current limiting. Although the passive fault current limiting can indeed limit the fault current, it has the following obvious short-comings: 1) it does not fully utilize all of the iron cores for limiting the fault current, thus demands heavier iron cores and larger size of the AC winding to produce the desired current limiting effects; 2) the DC side has to be subject to high inductive voltage during the state of the fault current limiting; and 3) the DC power supply must be a constant power supply. Otherwise, the efficiency of current liming will be reduced if there is interference in the power supply.
Furthermore, for the superconductive fault current limiters on a high voltage grid, AC windings are at high voltage level while iron cores and superconductive magnets are at low voltage level. Safe insulation distances are required between AC windings, between AC windings and the iron cores, and between superconductive magnets. For this reason, the loose coupling structure shown in FIG. 5 is generally adopted, that is, superconductive magnet 2 and the AC winding 5 are on different iron core posts of the same iron core window. Since the iron cores operate in the nonlinear segment of the B-H magnetization curve of iron core materials for a certain time period or all time periods, which falls in the range of saturation or deep saturation. Therefore, in the loose coupling structure, magnetic leakage is inevitable. This phenomenon becomes aggravated as the degree of saturation increases. FIG. 6 shows the schematic diagram of the rectangular-shape iron core excitation of prior art. The iron core part wound by the superconductive magnet 2 is called as an excitation segment 41. The iron core part wound by the AC winding 5 is called as a working segment 42. The other part connected with the excitation segment 41 and the working segment 42 is called as a conduction segment 43. When the excitation current in the superconductive magnet 2 is low, the magnetic field in the iron cores is weak. The iron cores are in the unsaturated state, with lower magnetic leakage flux. The magnetic flux in the working segment 42 and the excitation segment 41 are approximately same. If the excitation current is increased, the magnetic leakage increases. Because the magnetic leakage increases, the ratio of magnetic potential on the excitation segment 41 to the total magnetic potential produced by windings is also increasing. As a result, the magnetic potential on the working segment 42 reduces relative to that on the excitation segment 41. It is needed to increase excitation current to reach a certain degree of saturation, and it is difficult to reach a certain degree of saturation even with increased excitation current.
Consequently, the traditional core-saturated superconductive fault current limiter has been considered as a current limiting technology with higher cost, heavier weight and the required power supply technology is harder to implement and is impractical in reality.